First Per-6-O-tritylation of Cyclodextrins

Multicharged cyclodextrin supramolecular assemblies

Multicharged cyclodextrin (CD) supramolecular assemblies, including those based on positively/negatively charged modified mono-6-deoxy-CDs, per-6-deoxy-CDs, and random 2,3,6-deoxy-CDs, as well as parent CDs binding positively/negatively charged guests, have been extensively applied in chemistry, materials science, medicine, biological science, catalysis, and other fields. In this review, we primarily focus on summarizing the recent advances in positively/negatively charged CDs and parent CDs encapsulating positively/negatively charged guests, especially the construction process of supramolecular assemblies and their applications. Compared with uncharged CDs, multicharged CDs display remarkably high antiviral and antibacterial activity as well as efficient protein fibrosis inhibition. Meanwhile, charged CDs can interact with oppositely charged dyes, drugs, polymers, and biomacromolecules to achieve effective encapsulation and aggregation. Consequently, multicharged CD supramolecular assemblies show great advantages in improving drug-delivery efficiency, the luminescence properties of materials, molecular recognition and imaging, and the toughness of supramolecular hydrogels, in addition to enabling the construction of multistimuli-responsive assemblies. These features are anticipated to not only promote the development of CD-based supramolecular chemistry but also contribute to the rapid exploitation of these assemblies in diverse interdisciplinary applications.

Dually acid- and GSH-triggered bis(β-cyclodextrin) as drugs delivery nanoplatform for effective anticancer monotherapy

The intrinsic poor solubility and limited load capacity of β-cyclodextrins (β-CDs) results in reduced bioavailability, rendering the material unsuitable in complex biological environments. In this work, a pair of β-CDs was methylated and covalently linked with acid-sensitive acylhydrazone and GSH-sensitive disulfide bonds to ensure a precise drug release pattern. The hydrophobic anticancer drug doxorubicin (Dox) was encapsulated inside the hydrophobic core of bis(β-CD) via hydrophobic association with loading capacity of 24% in weight and a hydrodynamic size of about 100 nm. When exposed to acidic and reductive environments, the acylhydrazone and disulfide bonds were found to be cleaved, resulting in Dox release. Using fluorescence imaging and flow cytometry analysis, the designed bis(β-CD) were determined to activate the drug release behavior by specific intracellular stimuli (pH and GSH). In vivo studies demonstrated specific drug delivery characteristics and controlled drug release behaviors in the tumor sites, giving rise to high antitumor activity and low toxicity. Taken in concert, this dual stimuli-responsive bis(β-CD) with superior amphiphilicity and biocompatibility features showed great potential for future clinical applications.

Transition-Metal-Free Synthesis of C-Glycosylated Phenanthridines via K2S2O8-Mediated Oxidative Radical Decarboxylation of Uronic Acids

We have developed an efficient protocol for the synthesis of C-glycosylated phenanthridines. Tetrafuranos-4-yl and pentapyranos-5-yl radicals, generated from K₂S₂O₈-mediated oxidative decarboxylation of furan- and pyranuronic acids, undergo attack to 2-isocyanodiphenyls and ensuing homolytic aromatic substitution to provide diverse C-glycosylated phenanthridines in satisfactory yields without resort to transition metals. This reaction tolerates various functional groups, and enables ready synthesis of complex oligosaccharide-based phenanthridines. The C-glycosylated phenanthridine derived from β-cyclodextrin has been prepared, which might be potential in medicinal and biological chemistry due to its flexible conformation.

Radical-Mediated C-H Functionalization: A Strategy for Access to Modified Cyclodextrins

A simple and efficient radical C-H functionalization to access modified cyclodextrins (CDs) has been developed. The well-defined conformation of glycosidic and aglyconic bonds in α-, β-, and γ-CDs favors the intramolecular 1,8-hydrogen atom transfer (HAT) promoted by the 6^I-O-yl radical, which abstracts regioselectively the hydrogen at C5^II of the contiguous pyranose. The C5^II-radical evolves by a polar crossover mechanism to a stable 1,3,5-trioxocane ring between two adjacent glucoses or alternatively triggers the inversion of one α-d-glucose into a 5-C-acetoxy-β-l-idose unit possessing a ¹C₄ conformation. The 6^I,IV- and 6^I,III-diols of α- and β-CDs behave similarly to the monoalcohols, forming mostly compounds originating from two 1,8-HAT consecutive processes. In the case of 6^I,II-diols the proximity of the two 6-O-yl radicals in adjacent sugar units allows the formation of unique lactone rings within the CD framework via a 1,8-HAT-β-scission tandem mechanism. X-ray diffraction carried out on the crystalline 1,4-bis(trioxocane)-α-CD derivative shows a severe distortion toward a narrower elliptical shape for the primary face.

Cyanoethylation of cyclodextrin derivatives

The synthesis of novel cyanoethylated cyclodextrin derivatives from previously prepared intermediates is described. It was found the alcohols at the primary face of cyclodextrins readily react to add to acrylonitrile, but similar additions from hydroxyl groups of the secondary face appear to be more difficult. The obtained cyanides could be reduced to form the corresponding amines.

Qualitative evaluation of regioselectivity in the formation of di- and tri-6-O-tritylates of α-cyclodextrin

The quantitative analysis of reaction products showed that the reaction of 6^A,6^D-di-O-trityl-α-cyclodextrin (CD), AD-isomer, with trityl chloride in pyridine at 55 °C gave 6^A,6^B,6^E-tri-O-trityl-α-CD, the amount of which was only 25% of that of simultaneously formed 6^A,6^B,6^D-tri-O-trityl-α-CD. This indicates that the bulky trityl groups of glucopyranose-A and -D (Glu-A and -D, respectively) in the AD-isomer mainly retard the additional tritylation of the C(6)-OH of the adjacent glucopyranoses in a counter-clockwise direction (Glu-F and -C, respectively). ¹H NMR spectra of the AD-isomer showed that the O(6)-H and C(6)-H signals of Glu-C and -F are shifted upfield due to the ring current of the trityl groups. Thus, it is concluded that the bulky trityl groups on Glu-A and Glu-D are oriented to Glu-F and Glu-C, respectively, and sterically retard additional tritylation on Glu-F and Glu-C. Similar steric hindrance was also observed in the additional tritylations of mono-6-O-trityl-α-CD, 6^A,6^B-di- and 6^A,6^C-di-O-trityl-α-CD’s.